Ultrasonic scanners for detecting blood flow based on the Doppler effect are well known. Such systems operate by actuating an ultrasonic transducer array to transmit ultrasonic waves into the object and receiving ultrasonic echoes backscattered from the object. In the measurement of blood flow characteristics, returning ultrasonic waves are compared to a frequency reference to determine the frequency shift imparted to the returning waves by flowing scatterers such as blood cells. This frequency shift translates into the velocity of the blood flow.
In state-of-the-art ultrasonic scanners, the pulsed or continuous wave (CW) Doppler waveform is computed and displayed in real-time as a gray-scale spectrogram of velocity versus time with the gray-scale intensity (or color) modulated by the spectral power. The data for each spectral line comprises a multiplicity of frequency data bins for different frequency intervals, the spectral power data in each bin for a respective spectral line being displayed in a respective pixel of a respective column of pixels on the display monitor. Each spectral line represents an instantaneous measurement of blood flow.
FIG. 1 is a block diagram of the basic signal processing chain in a conventional spectral Doppler mode. An ultrasound transducer array 2 is activated to transmit by a transmit ultrasound burst of length P which is fired repeatedly at a pulse repetition frequency (PRF). The PRF is typically in the kilohertz range. The return RF signals are detected by the transducer elements and received by the respective receive channels in the beamformer 4. The beamformer sums the delayed channels data and outputs either RF or in-phase and quadrature (I/Q) data. The latter alternative is illustrated in FIG. 1.
The output of the beamformer is shifted in frequency by a demodulator 6. One way of achieving this is to multiply the input signal by a complex sinusoidal e.sup.i2.pi..function.dt, where .function..sub.d is the frequency shift required. The demodulated I/Q components are integrated (summed) over a specific time interval T and then sampled at the PRF by a so-called "sum & dump" block 8. The summing interval and transmit burst length together define the length of the sample volume as specified by the user. The "sum and dump" operation effectively yields the Doppler signal backscattered from the sample volume. The resultant "slow time" Doppler signal samples are passed through a wall filter 10 which rejects any clutter corresponding to stationary or very slow-moving tissue. The filtered output is then fed into a spectrum analyzer 12, which typically takes Fast Fourier Transforms (FFTs) over a moving time window of 64 to 128 samples. Each FFT power spectrum is compressed (block 14) for display on a monitor 16 as a single spectral line at a particular time point in the Doppler velocity (frequency) versus time spectrogram.
One of the primary advantages of Doppler ultrasound is that it can provide noninvasive and quantitative measurements of blood flow in vessels. Given the angle .theta. between the insonifying beam and the flow axis, which is usually specified by rotating a cursor line in the B-mode image of a duplex scan, the magnitude of the velocity vector can be determined by the standard Doppler equation: EQU v=cf.sub.d /(2f.sub.0 cos .theta.)
where c is the speed of sound in blood, .function..sub.0 is the transmit frequency and .function..sub.d is the motion-induced Doppler frequency shift in the backscattered ultrasound. In practice an intensity-modulated Doppler frequency versus time spectrogram is displayed since the Doppler sample volume or range cell generally contains a distribution of velocities that can vary with time.
The summing interval T and the transmit burst length P together define the axial sensitivity profile of the user-select sample volume. In other words, the "sum & dump" operation yields the Doppler signal back-scattered from the sample volume. The summer, which is often referred to as the "range gate," is essentially a moving averager. This implies that the duration of the Doppler sensitivity interval is given by the convolution of the transmit burst and the range gate, as illustrated in FIG. 2. The axial length of the sample volume is then given by c(P+T)/2. For the purpose of this analysis, one can ignore the finite-bandwidth transducer effect on the idealized axial sensitivity profile of FIG. 2.
For a given Doppler scan geometry and system noise floor, the sensitivity to blood flow generally depends on the size of the sample volume (how much blood is insonified), the amplitude of the transmit burst (strength of insonification) and the P/T ratio. In accordance with optimal detection theory, for a given acoustic dosage the signal-to-noise ratio (SNR) is maximized when P/T=1, i.e., when the range gate is matched to the transmit burst. As indicated by the dashed lines in FIG. 2, this results in a triangular sample volume shape with maximum peak amplitude.
If a large sample volume is used to interrogate a shallow vessel, the parameters P, T and PRF can be so large (relative to B-mode) that Doppler sensitivity is not an issue. In fact, in such cases the Doppler sensitivity is probably already at its maximum allowed by the regulation dosage. In general, there is room for sensitivity improvement only in cases where the acoustic dosage is below regulation limit. For example, if one wants to interrogate a deep-lying vessel using a small sample volume, the longer round trip time will automatically limit the PRF to lower values. Together with an increased tissue attenuation factor, the acoustic dosage can fall below regulation limit at the sample volume position. If the user selects a small sample volume because he or she wants to avoid pickup from adjacent vessels or clutter sources, or to just examine a small region of interest (ROI) within the vessel of interest, then the transmit burst length P must be limited as well. To maximize the acoustic dosage, the transmit amplitude can be increased, but this may not always be possible due to the finite voltage limit of the pulser. In the worst cases, the flow signal may be too weak to be detected. In practice, this may force the user to forgo spatial resolution by increasing the sample volume size to 5 mm or longer in order to transmit a burst having increased length P and higher power (and using a longer range gate T).
Thus, there is a need for a method of improving pulsed Doppler sensitivity and/or sample volume resolution in such cases.